The present invention relates to a high-quality image display device and more particular, to an image display device of a light-emitting flat-panel type such as organic electro-luminescence.
There are various types of such flat-panel type image display devices including a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic electro-luminescence (which will also be referred to merely as the organic EL, hereinafter) device, which go into actual use or are still in the research stage of actual use. Of these flat panel type image display devices, self light-emitting and light-emitting flat panel types, where pixel itself emits light, receive much attention. In the LCD or organic EL devices having a pixel circuit of thin-film transistors (TFTs) each formed for each pixel, an active type has been predominantly used.
Explanation will be made as to the arrangement and exemplary operation of a prior art light-emitting flat panel (which will also be referred to merely as the light-emitting display device, hereinafter) as an image display device, with reference to FIGS. 13, 14, and 15. FIG. 13 shows a structure of a prior art light-emitting display device. In the drawing, pixels 201 are provided in a display zone 200 in form of a matrix having rows and columns. And a signal line 202, a gate line 203 and a power line 204 are connected to each pixel 201. Many of the pixels 201 are actually provided in the display zone 200, but only one of the pixels is shown for simplicity of the drawing. The signal line 202 is connected at its one end with a signal voltage input circuit 206, and the gate line 203 is connected at its one end with a shift register circuit 205. The power line 204 is connected at its one end with a power supply circuit 208 via a current measuring circuit 207.
FIG. 14 shows a diagram for explaining an exemplary structure of the pixel 201 in FIG. 13. One end of a first thin-film transistor (pixel TFT) 210 is connected to the signal line 202. A gate of the pixel TFT 210 is connected to the gate line 203, and the other end of the pixel TFT 210 is connected to a gate of a second thin-film transistor (driving TFT) 212. One end of a capacitance 211 is connected to the gate of the driving TFT 212, and the other end of the capacitance 211 is connected to the power line 204 commonly together with one end of the driving TFT 212. The other end of the driving TFT 212 is connected to one end of a light emitting element 213 (organic EL element in the illustrated example), and the other end of the light emitting element 213 is connected to a common grounding terminal 214.
Explanation will next be made as to the operation of the image display device shown in FIGS. 13 and 14. In a regular image display mode, the signal voltage input circuit 206 sequentially outputs a signal voltage to the signal lines 202. In synchronism with it, the shift register circuit 205 continues to select and scan the pixel 201 for the signal voltage to be written therein. During the above operation, power is supplied from the power supply circuit 208 to the power lines 204. When the gate line 203 of the pixel 201 is selected and the pixel TFT 210 is turned ON during the output of the signal voltage to the signal line 202, the signal voltage is written in the capacitance 211. Since the written signal voltage is still stored in the capacitance 211 even after the pixel TFT 210 is turned off, the written signal voltage is always input to the driving TFT 212. This results in that the driving TFT 212 inputs a drive current corresponding to the written signal voltage to the light emitting element 213, and the light emitting element 213 emits light with a brightness corresponding to the signal voltage.
Ideally, the image display should be realized through the above operation without any trouble, but it actually involves a problem that luminous brightness gradually varies with deterioration of the light emitting element 213 with time passage. Since the degree of such deterioration of the light emitting element 213 with time varies from pixel to pixel, the element deterioration generates a fixed burned pattern of noise in the displayed image. To avoid this, the prior art is arranged so that a deterioration in each pixel is measured and the measured deterioration is fed back to the display signal voltage to cancel the aforementioned fixed pattern of noise.
Explanation will be made as to the operation of the prior art image display device of FIG. 13 when a deterioration in each pixel is measured. FIG. 15 shows a diagram for explaining a sequence when a drive current is measured for each pixel row. First, a black level is written into all the pixels 201 by the signal voltage input circuit 206 over a period of one frame.
Thereafter, as the shift register circuit 205 sequentially selects each pixel row, a white level is written by the signal voltage input circuit 206, a drive current for each pixel is measured by the current measuring circuit 207, and a black level is written by the signal voltage input circuit 206. These operations are repeated. Through the repeated operations, the drive current characteristics of all the pixels 201 are measured.
On the basis of a change in the drive current characteristic thus obtained, a degree of deterioration of the light emitting element 213 at each pixel is acquired. The above fixed pattern of noise can be canceled by feeding the acquired result back to the signal voltage. Such a prior art is described in detail, for example, JP-A-2002-278514 and JP-A-2002-341825. Prior arts associated with a pixel circuit in an embodiment to be explained later are disclosed in JP-A-2003-5709 and JP-A-2003-122301.
In the aforementioned prior art, for the purpose of measuring a drive current characteristic corresponding to one pixel row, three sequences (1) to (3) are required. That is, (1) writing of the black and then white level to all the pixels by the signal voltage input circuit 206, (2) measurement of the drive current for each pixel by the current measuring circuit 207, and (3) writing of the black level by the signal voltage input circuit 206, are required. Since accurate writing to the signal line 202 and/or the power line 204 is carried out in any of the three operations, a predetermined writing time becomes necessary. For this reason, for measuring the drive current characteristics of all the pixels, a time as relatively long as one frame or more is required. Thus it is difficult to cancel a variation in the characteristic on a real time basis while a motion image is displayed.
The deterioration of the light emitting element with time advances slowly. Thus the need of measuring a characteristic change on a real time basis should be eliminated. However, from the fact that the characteristic of the light emitting element is sensitive to temperature, we noticed a problem that the characteristic varies with heat generated by the element itself on a real time basis. Since such characteristic variation caused by the temperature change disappears in a certain time, it affects the image quality in the form of a sort of long-time after-image, thus deteriorating the stability of the luminous brightness.